Pakettic

Pakettic is a command-line tool for minifying and compressing TIC-80 fantasy console carts. The tool is written in Python (3.9+) and used especially for sizecoding. It compresses existing carts approximately ~1.2% better than best alternatives, and by using its magic comments, pakettic might find code that compresses even better.

Installation

Installing with pip:

$ pip install pakettic

Installing the latest main branch from GitHub:

$ pip install git+https://github.com/vsariola/pakettic.git@main

Installing a checked out version of the repository:

$ pip install -e path/to/pakettic

Installing a checked out version of the repository using poetry for a nice virtual environment with locked dependencies (run inside the pakettic folder):

$ poetry install

Usage

To compress a cart, run:

$ pakettic path/to/cart.tic

If your PATH is not configured to include pip installed executables, you can use

$ python -m pakettic path/to/cart.tic

If you installed using poetry into a virtual environment, you need to prepend poetry run before every command e.g.

$ poetry run pakettic path/to/cart.tic

Pakettic supports both .tic and .lua carts. Multiple input files may be defined. Input files are globbed, so ?, *, and ** work as wildcards for a single character, multiple characters and a directory, respectively.

For a full list of command line options, see:

$ pakettic --help

See also tips for command line arguments

Running all tests:

$ poetry run python -m unittest discover -s tests

Features

Pakettic first parses the LUA-script to an abstract syntax tree, and then uses a local optimization algorithm (simulated annealing, late acceptance hill climbing or its variant diversified late acceptance search) to randomly mutate the syntax tree & see if it compresses better. Implemented mutations include:

• shortening variable names
• flipping comparisons >, <, >=, <=, ~=, and ==
• reordering arithmetic operators +, -, * and / and bit logic operators &, ~ and |
• converting a^2 into a*a and vice versa
• using either single or double quotes for all strings
• converting whole hexadecimals into decimals
• convert for a,b,1 do into for a,b do and vice versa
• reordering statements: statements that can be reordered are marked with magic comments
• alternative expressions: alternatives are marked with magic comments
• folding constant expressions

Internally, pakettic uses zopfli for the compression.

load'<code>' is parsed as function(...)<code>end so you can easily recompress already compressed carts. Conversely, function()<code>end or function(...)<code>end is replaced with load'<code>' during compression.

However, pakettic does not convert functions with parameters. In particular, pakettic does not automatically convert function SCN(x)<code>end into SCN=load'x=...<code>', because they are not semantically identical: in the load version, x is now global and thus could trash a global variable, unintentionally breaking the cart. To make SCN compress nicely, you have to write it as function SCN(...)x=...<code>end, taking responsibility for x not overwriting anything important.

Unnecessary parentheses are removed from expressions so you do not have to worry about those.

Magic comments

Reorderable statements

The algorithm will try to reorder statements between --{ and --}. For example:

 --{
 a="hello"
 b="world"
 --}

will try both a="hello"b="world" and b="world"a="hello" to see which compresses better.

Notice that only complete statements can be reordered. Thus, this will NOT work:

 --{
 for x=0,239 do
  for y=0,135 do
 --}
  end
 end

A good rule of thumb is that you should be able to replace --{ and --} with do and end, respectively, and still have valid code.

Statements between --{! and --} are not ordered, so you can make blocks of statements that are kept in order within a pair of --{ and --} tags.

Alternative expressions

There is a special --| operator that allows alternative expressions to be tested, to see if they compress better. For example: 5--|4--|6 means that the algorithm will try 4 and 6 in place of the 5. This will naturally show up as a comment in LUA so you will have to continue the expression on next line if this is in the middle of an expression. --| has the lowest precedence, even lower than ^, so put parentheses if you want to try more complicated expressions e.g. (x//256)--|(x>>8)

Debug code

Pakettic treats --![ and --!] as multiline comment tags, while LUA treats these as single line comments. Useful for including debug code in the unpacked intro: the code will not be included in the packed cart.

Tips for command line arguments

• The Zopfli compression level can be set with -z<level>, with level ranging from 0 to 5. When developing, start with -z0 for fast optimization, and only increase when necessary e.g. when you are just a few bytes over the limit. The default Zopfli-level is 2.
• The algorithm uses a pseudorandom generator. Sometimes using a different seed finds a few byte better or worse solution. Use command line argument --seed to try different seeds.
• Similarly, different optimization heuristics produce slightly different results. Try different heuristics e.g. with -alahc, -adlas or -aanneal.
• To avoid re-optimizing all the expressions every time, do a long optimization run, study the results and change your expressions to the forms that pack well. Set the number of steps with -s. Use command-line argument -p to always print a reasonably readable version of the best solution when one is found.
• By default, pakettic only includes CODE and DEFAULT chunks. DEFAULT indicates that before loading the cart, TIC-80 loads the default cart, setting default palette, waveforms etc. If you don't need the default values (e.g. you set the palette yourself), save one byte by only including CODE chunk in the cart: -ccode
• Working on a tweet-cart? Use -l to output LUA carts, which are uncompressed. The optimization algorithm then just optimizes the uncompressed size of the code.

Known issues

• At the moment, all the branches of swappable operators are assumed to be without side effects. If they have side-effects, the swapping might inadvertedly swap the execution order of the two branches.
• The parser can crash with large carts. Carts in the size coding range (few thousand characters) do not seem to cause problems, but crashes have been observed parsing carts with tens of thousands of code characters. This may be related to how the pyparsing grammar is defined, which could result in highly recursive parsing and eventually stack overflows.

Credits

Code contributors: Veikko Sariola/pestis, wojciech-graj, koorogi

Test corpus contributors: psenough, ilmenit, gigabates, gasman, pellicus, luchak.

License

MIT

The test corpus carts have their own licenses, see the license files in the subdirectories of the corpus directory.